1. Field of the Invention
The invention relates to a process for sealing a filled sack made of thermoplastic plastic and provided with gussets in which the open side of the sack is pulled tight and closed with a crosswise seal.
2. Description of the Related Art
Devices for closing sacks in accordance with this process are known, for example, from BP 0 439 789 B1, 93 01 355 U1, and DE 199 36 660 A1. Using the known devices, the crosswise seal to close the sack at the top is produced by heated sealing jaws, i.e., by means of a thermal sealing process. In filling sacks with bulk material, it is impossible to prevent dust and residual filling from being deposited in the region of the sack through which the crosswise seal closing the sack is later formed, which prevents the creation of a good and tight seal.
The object of the invention is therefore to create a process of the type mentioned at the outset that allows a good and tight closing of the sack by means of a crosswise seal even when dust or other impurities have been deposited in the region of the opening to be closed by means of the crosswise seal.
This object is attained according to the invention in that, before the sack is filled, the lateral regions of the crosswise seal including the gussets are formed on the sack, which is still lying flat, up to the transitional region to the two-layer center region in the first sealing sequence and the center region of the crosswise seal is produced in the second sealing sequence after the sack has been filled.
Preferably, the first sealing sequence is performed using a thermal sealing process and the second sealing sequence is performed using an ultrasound sealing process.
In the process according to the invention, the parts of the crosswise seal sealing the sack that include the four-layer region of the gussets are produced using a thermal sealing process that is particularly suitable for sealing four layers and also allows a sealing of the transitional region from the four-layer gusset region and the two-layer center region. This sealing of the gusset region occurs by means of a thermal sealing process and is performed at a point in time at which the sack is not yet filled and thus the regions to be sealed with one another are not yet contaminated by dust or residual filling. After the formation of the regions of the crosswise seal including the gussets by means of a thermal sealing process, i.e., for example, by means of conventional sealing jaws, the sack can be held under the feed opening of a filling trunk by holding grippers. After the sack has been filled, the upper region is then pulled tight so that the walls of the two-layer region are resting against one another. According to the invention, this center region is then closed by means of a seal produced in an ultrasound sealing process. The advantage of sealing the center region in an ultrasound process lies in the fact that impurities are driven from the seal zone so that the crosswise seal can be produced in a problem-free manner between the pre-existing seals produced earlier that include the gussets.
The sealing of film layers of thermoplastic plastic resting against one another in an ultrasound sealing process is known, for example, from DE 197 12 498 C2.
It is useful for the seal produced in the ultrasound sealing process to overlap with the seals that were produced in a thermal sealing process, which have been extended past the gussets.
An exemplary embodiment of the invention shall be explained in greater detail in the following with reference to the drawings, which show: